test.s:4: Error: suffix or operands invalid for `out'

пишу:
.globl _start
_start:
mov 0,%ax
out 0x43,%al
in %al,0x40
in %al,0x40
компилирую:
gcc -c test.s
test.s: Assembler messages:
test.s:4: Error: suffix or operands invalid for `out'
test.s:5: Error: suffix or operands invalid for `in'
test.s:6: Error: suffix or operands invalid for `in'
В чём дело?